If you are off on a shore excursion, the buses will be right at the pier regardless of where your ship ties up. If you want to explore on your own, you likely will not find the walk from the pier to the sights to be long.
Be aware though, on busy days when there are many ships in port, someone will wind up with the short straw and your ship may not have a berth to tie up at. In this case, you will be shuttled to shore on one of the ship's tender boats. Usually your cruise itinerary will tell you if you will be tendered. Regardless, this is a rare occurrence.
For the major ports of call, this is most likely where you will dock if you are on a ship of one of the major cruise lines:
1. Icy Strait Point: you will always be tendered to shore as the cruise ship dock has not been put in yet. The tender ride is 10 - 15 minutes in length.
2. Juneau: has a downtown dock for most cruise lines - while this is at the south end of town, it is also where most of the shopping is. Princess ships tie up five to ten minutes further south at their own dock. There are a couple of shops located adjacent to the dock here as well.
3. Ketchikan: again, there is a downtown dock for all cruise lines. You can literally step off the ship and start shopping. The only exception is if the dock space is all taken and your ship has to tender its passengers to shore.
4. Seward: the docks are at the north end of town and it is a bit of a walk to some of the prime sites such as the Marine Rehabilitation Center. If you don't mind the 20 - 30 minute walk to get to the south end of Seward, the town is still fairly compact. As this is usually the start or end to a cruise, most passengers don't spend much time looking around Seward.
5. Skagway: Princess ships dock at the railway pier at the east side of the harbour. As the name implies, the Yukon Whitepass Railroad line is 30 meters from the gangway. It is about a 10-20 minute walk to town from the ship depending on how fast you are and whether your ship is number 1 or 2 at the dock. Other ships dock to the west of the harbour, have their own railroad service and are a similar distance from town.
6. Sitka: if you are lucky enough to visit Sitka, you will always be tendered to shore as there is no dock capable of handling a cruise ship. The tender ride to shore is about 15 minutes.
7. Vancouver: ships dock either at Canada Place in downtown Vancouver or further east, at the Ballantyne Cruise Terminal. Both are excellent facilities and easy to reach by taxi from the airport or your hotel.
8. Whittier: the ship docks just east of the access tunnel to the town. Shops, such as they are, are a few hundred meters from the dock. This usually isn't a worry as this is a terminus or start to a cruise, not a place that passengers usually spend much time exploring